As explained in U.S. Pat. No. 6,209,726, vibratory screening machines are well known in the art and are used in a variety of situations where it is desired to remove suspended solids from a slurry. A common situation where these are used are in the drilling of oil and gas wells where drilled solids are circulated to the surface by drilling mud. Although the screen assembly of this invention has utility in other applications, it will be described in connection with the removal of drilled solids from drilling mud where the vibratory screening machines are called shale shakers.
Conventional vibratory screening machines include a rectangular screen assembly that is vibrated. The liquid slurry is discharged onto the screen which is typically inclined so solids in the slurry, larger than the screen size, collect on top of the screen and migrate toward the discharge end, typically off one of the longer sides. Solids in the slurry smaller than the screen size pass with the liquid through the screen.
Early shale shakers incorporated a single inclined vibrating layer of hardware cloth having a mesh opening of ¼-⅜″. Drilling mud coming from the well discharged onto the inclined screen. Large shale particles collected on top of the hardware cloth and travelled down the incline into a shale pit. The liquid drilling mud and the bulk of the entrained solids passed into the mud system.
Substantial improvements have been made in vibratory screening machines so very small solids are now capable of being removed from hot drilling mud streams emitting from wells being drilled at substantial depths in the earth. Larger drilling rigs are equipped with sophisticated mud systems that treat the drilling mud to perform its various tasks. A typical large drilling rig includes a shale shaker mounted on a mud tank so the removed solids are discharged into a shale pit adjacent the mud tank and the liquid mud passing through the shale shaker falls into the mud tank where it is treated by monitoring various properties, by adding various chemicals and by using other solids removal techniques such as cyclones, centrifuges and the like.
When starting the drilling of a land based well, however deep, the surface hole is typically drilled with a combination of water and bentonite gel which combines with drilled solids to make a native drilling mud. This type mud is not expensive and is not treated in a costly manner. When drilling surface hole, the screen assemblies on the shake shaker are selected to have rather large mesh so that only fairly large solids are removed from the mud stream. Because the screen assemblies have large mesh screen, they have screen wire of substantial diameter and are accordingly robust and operate satisfactorily for substantial lengths of time.
As the well is deepened, the drilling mud is treated with more expensive chemicals and more care is taken to control the amount and size of solids in the recirculated mud. In the drilling of a typical deep well, one or more strings of intermediate pipe are cemented in the hole to provide protection against blow outs. Typically, more expensive mud types are used following the setting of intermediate strings. For example, it is common in parts of South Texas to drill a well with a water based gel mud until an intermediate string of pipe is set and then change over to an oil based invert emulsion. These oil based emulsions are considerably more expensive than the water based mud used to drill the shallower part of the hole. Considerably more care is taken to remove solids from more expensive muds, of which oil based invert emulsions are typical.
The screen assemblies in shale shakers are accordingly changed during drilling of wells to provide larger mesh, less expensive, more durable screen assemblies when drilling the shallow part of the hole and smaller mesh, more expensive, less durable screen assemblies when drilling the deeper part of the hole. The trend, over time, has been to use finer and finer mesh screens when using expensive muds. The finest screen mesh commonly presently employed in screen assemblies is on the order of 210-250 mesh, which means there are 210-250 strands of wire per inch. A conventional 210 mesh screen will remove solids larger than 74 microns from drilling mud. Occasionally, mesh sizes up to 280 strands of wire per inch are used in special situations, such as drilling with brine.
There are presently several types of screen assemblies employed in sophisticated vibratory screening machines used as shale shakers. One type employs a metal plate as a support for the screens where the screens are bonded in one fashion or other to the metal plate. A second type is shown in U.S. Pat. No. 6,209,726 and employs four screens and a perforate plastic mesh that bonds the screens together.
As shown in FIG. 1, a third type prior art screen assembly 10 includes a rectangular metal frame 12, a fine mesh top screen 14, a blinding screen 16, a plastic grid or mesh 18 and a load bearing screen 20. The frame 12 includes a rectangle of tubular members 22 and a series of short tubular members 24 spanning the short dimension of the screen assembly 10. The plastic mesh 18 includes a peripheral section 26 overlying the rectangular members 22 and a series of strips 28 overlying the short tubular members 24 an perforate panels 30 spanning between the strips 28 and members 24. The openings 32 in the perforate panels 30 may vary somewhat but are almost always between 1″ square and 2″ square, meaning that the openings are square either 1″ or 2″ on a side. The frame 12, the screens 14, 16, 20 and the plastic mesh 18 are put into a heated press where the temperature softens the plastic mesh 18 and an applied pressure squeezes the screens 14, 16, 20 into the plastic mesh 18, or vice versa, thereby bonding the layers together to provide a unitary structure. When inspecting a manufactured screen assembly 10, it is difficult to determine whether the plastic mesh 20 started out between the screens 14, 16 or between the screens 16, 20 or between the screen 20 and the frame 12 because the plastic is so completely intermeshed with the screens. Screen assemblies of this construction have proved suitable for use in the shallower part of hole where the upper screen 14 is on the order of 140 mesh or coarser.
As shown in FIG. 2, another prior art screen assembly 34 includes a rectangular frame 36 having made of tubular members 38 and including intermediate members 40 parallel to the short dimension of the frame 36. A bead 42 of epoxy is applied to the tubular members 38, 40. A fine mesh screen 44, a blinding screen 46 and a support screen 48 are tensioned and then applied to the epoxied frame 36 with a suitable amount of pressure so the epoxy 42 becomes dispersed through the screens 46, 48 and sets up.
Disclosures of interest relative to this invention are found in U.S. Pat. Nos. 4,033,865; 4,575,451; 5,221,008; 5,330,057; 5,417,859 and 5,673,797.